Geoff Hall has devoted much of his career to development of electronic instrumentation for particle physics experiments, which in the last thirty years has focused on the CERN LHC.
After his PhD, he joined an experiment at SLAC using a rapid-cycling bubble chamber with cameras triggered using information from external detectors, processed using a fast microprocessor. After the discovery of charmed quarks in 1974, there was intense interest in directly measuring charmed particle lifetimes, which was achieved in the BC72 experiment. Hall designed gas Cherenkov counters for particle identification, developing 3D ray tracing software for design. Over two million pictures were taken, leading to a sample of about 100 charm decays.
Charmed quark studies required greatly increased event samples, only feasible by developing new triggered electronic detectors with few µm resolution. Imperial College took up this challenge, led by David Websdale, collaborating with a UK company to manufacture silicon microstrip sensors for NA14.
During subsequent years other prototype detectors were developed, using optical masks made by electron beam lithography in UK facilities, including photodiodes for scintillator readout for UA1 and sensors for luminosity monitoring in the ALEPH experiment. Silicon drift detectors were built, optimised for UV sensitivity and successfully deployed at CERN in a liquid xenon calorimeter cell.
Other studies included evaluation of silicon microstrip technology for biomedical purposes and prototypes for autoradiographic DNA sequencing using beta emitters. The potential of x-ray sensitive pixel detectors for synchrotron radiation detection was studied and prototypes demonstrated, in collaboration with Rutherford Appleton Laboratory engineers.
In the late 1980s, Hall joined a US collaboration to study the behaviour of neutron-irradiated silicon sensors and later co-founded the RD20 CERN R&D project to continue this work, and to develop suitable radiation hard ASICs for LHC.
During these studies, the phenomenon of “reverse annealing” was observed for the first time; irradiated sensors progressively become more p-type as radiation damage accumulates. It was shown that cooling the sensors was a means to control this phenomenon. Other work focused on the contributions of oxygen and carbon in defect evolution.
RD20 developed an early LHC readout chip, the APV6, for which Hall proposed an original method ("deconvolution") to process signals to achieve a fast response and reduce power. The APV25 chip, developed by Imperial and RAL, was the first major chip to successfully use 0.25µm CMOS technology for the LHC. About 75,000 of them have been operating in the CMS silicon tracker since 2008.
In 1992, Imperial joined the Compact Muon Solenoid collaboration to apply R&D developments to the proposed tracking detector, including optical fibre technology which was another vital technological innovation.
Hall has been a member of the CMS experiment since, working particularly on tracking detectors and readout electronics. Several tracker developments were applied to the electromagnetic calorimeter in 2002, and a new ASIC, the MGPA, was designed. The crystal calorimeter played a major role in the CMS discovery of the Higgs boson .
For more than a decade, attention has focused on extending the CMS lifetime. Radiation damage precludes survival of the tracking sensors, and a replacement detector must have greater granularity and higher speed readout, as well as increased radiation tolerance. It must also provide data to be used by the CMS trigger to select rare events efficiently. The Imperial group has contributed several unique ideas to make this possible. The first is to deploy double-layer silicon modules (“pT-modules”) to identify high transverse momentum tracks, to design front end electronics to provide this information, and an off-detector system to process it. The CBC ASIC, for microstrip readout and data transmission for the trigger was designed and is about to enter production.
In the CMS trigger, the FPGA technology originally pioneered for the tracker has become a strength of the Imperial group. In 2016, a team demonstrated the viability of FPGAs for sufficiently fast, efficient track reconstruction for the HL-LHC.
Hall shared the UK Institute of Physics 2004 Duddell medal and prize, for development of radiation hard analogue electronics for silicon detectors. The APV25 has been widely used in experiments throughout the world. Spin-offs from CMS work have been exploited in other experiments, such as UA9, where a high angular resolution microstrip telescope has been used to study crystal channeling and produced many original results.
He was awarded the UK Institute of Physics James Chadwick medal and prize in 2020 "for his pioneering work in developing silicon detectors and front-end electronics for particle physics experiments, especially in crucial radiation-hard applications, critical for the observation of the Higgs boson in 2012”.
He was elected as a Fellow of the Royal Society in 2021.
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